ANNEX O 

Methodology for Estimating Net Changes in Forest Carbon Stocks 

This annex presents a discussion of the methodology used to calculate net changes in carbon stocks in trees, 

understory, forest floor, down dead wood, forest soils, wood products and landfilled wood. More detailed 

discussions of selected topics may be found in the references cited in this annex. 

The details of carbon conversion factors and procedures for calculating net CO2 flux for forests are given in 

three steps. In addition, the USDA Forest Service forest sector modeling projection system is described briefly. 

Step 1: Estimate Forest Carbon Stocks and Net Changes in Forest Carbon Stocks 

Step 1a: Obtain Forest Inventory Data 

Forest survey data in the United States by broad forest type and region for 1987 and 1997 were obtained 

from USDA Forest Service, Forest Inventory and Analysis estimates of forest resources, published in Waddell et al. 

(1989) and Smith et al. (2001). The Forest Inventory and Analysis data include: (1) growing stock volume per acre 

by forest type (referred to hereinafter as “growing stock volumes”); and (2) area by timberland and other forest land, 

for general forest types by region (referred to hereinafter as “forest areas”). For 2001, the same variables were 

obtained from model results as described in Haynes (2002). (See Forest Sector Modeling Projection System below). 

This information was combined with separate estimates of carbon density (carbon mass per unit area) to estimate 

carbon stocks. 

Step 1b: Estimate Carbon in Living and Standing Dead Trees 

To estimate live tree biomass, equations that convert forest tree volumes to total live tree dry biomass 

(Smith et al. in press) were applied to the growing stock volumes by forest type and region (obtained in Step 1a). 

Tree biomass includes aboveground biomass and belowground biomass of coarse roots. The minimum sized tree is 

one-inch diameter at diameter breast height (1.3 meter). Trees less than one-inch diameter are counted as carbon in 

understory vegetation. Biomass estimates were divided by two to obtain estimates of carbon in living trees (i.e., it 

was assumed that dry biomass is 50 percent carbon). Standing dead tree biomass was calculated by applying 

equations that estimate biomass for standing dead trees (Smith et al., in press) from growing stock volumes. Again, 

standing dead tree biomass was divided by two to estimate carbon in standing dead trees. Table O-1 lists the 

average living and standing dead tree carbon densities by forest type, as calculated by applying the equations to the 

1997 data. 

Table O-1: Average U.S. Carbon Densities of Forest Components* (Metric Tons C/ha) 

Regiona/Forest Type 

Live and Standing Dead Tree Forest Floor Soil Organic 

Carbon Carbon Carbonb 

Eastern 

White-red-jack pine 

Spruce-fir 

Longleaf-slash pine 

Loblolly-shortleaf pine 

Oak-pine 

Oak-hickory 

Oak-gum-cypress 

Elm-ash-cottonwood 

Maple-beech-birch 

Aspen-birch 

Other forest types 

Nonstocked 

77.1 13.8 196.1 

59.8 40.2 192.9 

42.4 9.2 136.3 

49.3 9.1 91.7 

57.3 11.6 82.3 

76.3 6.6 85.0 

86.0 6.0 152.2 

67.6 23.0 118.1 

82.5 28.0 139.5 

56.0 7.6 237.0 

1.8 2.1 99.6 

3.7 3.5 99.6 

O-1
Western 

Douglas-fir 

Ponderosa pine

Western white pine

Fir-spruce 

Hemlock-Sitka spruce

Larch

Lodgepole pine 

Redwood 

Hardwoods

Other forest types

Pinyon-juniper 

Chaparral 

Nonstocked 

110.8 30.7 89.6 

66.3 20.3 70.4 

69.2 25.8 68.3 

113.0 37.4 137.5 

152.4 34.1 157.1 

97.0 30.2 65.6 

67.8 23.9 62.7 

186.6 26.9 85.8 

89.0 9.9 79.5 

55.4 28.2 90.1 

20.8 21.1 56.3 

17.5 25.7 58.7 

18.1 24.4 90.1 

* Based on 1997 data for major forest types of the conterminous United States. aEastern United States is defined as states east of, and including, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, and Texas. 

Western United States includes the remaining conterminous states. bSoil includes both mineral soils and organic soils (i.e., histosols); carbon densities are to a depth of 1 meter. 

Step 1c: Estimate Carbon in Understory Vegetation 

To estimate carbon in understory vegetation, equations based on Birdsey (1992) were applied to the 

database that was used to produce the compiled forest statistics in Smith et al. (2001). Understory vegetation is 

defined as all biomass of undergrowth plants in a forest, including woody shrubs and trees less than one-inch 

diameter, measured at breast height. A ratio of understory carbon to live tree carbon was calculated, and multiplied 

by 100 to calculate the percentage of carbon in the understory relative to that in live trees. These percentages were 

then averaged for each forest type in each region. This percentage was multiplied by the live tree carbon data in 

1987 and 1997 to calculate understory carbon. These percentages are given in Table O-2. This procedure was used 

instead of applying the Birdsey equations directly, because detailed databases are not available for inventory years 

prior to 1987. Using estimated average values results in consistent historical carbon estimates for all survey years. 

Table O-2: Ratios of Understory and Down Dead Wood Carbon to Live Tree Carbon* (Percent) 

Ratio of Understory Ratio of Down Dead Wood Region/Forest Type Carbon to Live Tree Carbon to Live Tree Carbon Carbon 

Northeast 

White-red-jack pine 

Spruce-fir 

Longleaf-Slash pine 

Loblolly-shortleaf pine 

Oak-pine 

Oak-hickory 

Oak-gum-cypress 

Elm-ash-cottonwood 

Maple-beech-birch 

Aspen-birch 

Other Forest Types 

Nonstocked 

North Central 

White-red-jack pine 

Spruce-fir 

Longleaf-Slash pine 

Loblolly-shortleaf pine 

Oak-pine 

Oak-hickory 

Oak-gum-cypress 

Elm-ash-cottonwood 

Maple-beech-birch 

Aspen-birch 

Other Forest Types 

Nonstocked 

2.5 10.8 

2.6 13.3 

2.5 10.8 

2.5 10.8 

2.8 12.9 

2.4 10.9 

2.6 11.1 

2.6 11.1 

1.9 11.1 

2.7 13.6 

8.9 3.8 

8.9 3.8 

1.8 9.8 

2.2 17.4 

2.4 7.4 

2.4 7.4 

1.9 7.2 

2.3 9.6 

2.3 9.6 

2.2 10.8 

2.2 10.8 

2.8 13.3 

5.5 4.1 

5.5 4.1 

O-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001 
Southeast 

White-red-jack pine 

Spruce-fir 

Longleaf-Slash pine 

Loblolly-shortleaf pine 

Oak-pine 

Oak-hickory 

Oak-gum-cypress 

Elm-ash-cottonwood 

Maple-beech-birch 

Aspen-birch 

Other Forest Types 

Nonstocked 

South Central 

White-red-jack pine 

Spruce-fir 

Longleaf-Slash pine 

Loblolly-shortleaf pine 

Oak-pine 

Oak-hickory 

Oak-gum-cypress 

Elm-ash-cottonwood 

Maple-beech-birch 

Aspen-birch 

Other Forest Types 

Nonstocked 

Pacific Northwest Eastside of Cascades 

Douglas-fir 

Ponderosa Pine 

Western White Pine

Fir-Spruce 

Hemlock-Sitka spruce

Larch

Lodgepole pine 

Redwood 

Other hardwoods

Unclassified & other 

Pinyon-Juniper 

Chaparral 

Nonstocked 

Pacific Northwest Westside of Cascades 

Douglas-fir 

Ponderosa Pine 

Western White Pine

Fir-Spruce 

Hemlock-Sitka spruce

Larch

Lodgepole pine 

Redwood 

Other hardwoods

Unclassified & other 

Pinyon-Juniper 

Chaparral 

Nonstocked 

Rocky Mountain, Northern 

Douglas-fir 

Ponderosa Pine 

Western White Pine

Fir-Spruce 

Hemlock-Sitka spruce

Larch

Lodgepole pine 

6.8 23.9 

6.8 23.9 

6.8 23.9 

6.8 23.9 

5.2 28.0 

4.4 24.2 

2.2 21.8 

2.2 21.8 

4.4 24.2 

2.2 21.8 

11.9 2.0 

11.9 2.0 

5.9 18.6 

5.9 18.6 

5.9 18.6 

5.9 18.6 

4.4 17.3 

3.7 15.0 

2.2 15.7 

2.2 15.7 

3.7 15.0 

2.2 15.7 

16.9 1.7 

16.9 1.7 

1.6 10.0 

2.5 12.6 

1.6 10.0 

1.1 15.7 

1.6 10.0 

1.6 10.0 

2.6 21.3 

1.9 25.8 

1.4 8.9 

2.5 12.6 

10.7 3.7 

9.7 2.1 

9.7 2.1 

2.0 11.9 

2.5 18.1 

2.5 18.1 

1.0 13.7 

1.0 13.7 

2.0 11.9 

1.7 16.4 

2.0 11.9 

4.5 3.9 

1.7 16.4 

20.2 3.7 

14.2 3.0 

14.2 3.0 

2.6 19.2 

2.4 19.6 

2.2 9.7 

1.7 14.8 

2.0 18.7 

2.2 9.7 

2.4 19.6 

O-3
Redwood 2.2 9.7 

Other hardwoods 1.9 14.2 

Unclassified & other 2.2 9.7 

Pinyon-Juniper 16.1 3.2 

Chaparral 16.1 3.2 

Nonstocked 16.1 3.2 

Rocky Mountain, Southern 

Douglas-fir 2.8 19.4 

Ponderosa Pine 4.1 21.6 

Western White Pine 2.8 19.4 

Fir-Spruce 2.2 17.4 

Hemlock-Sitka spruce 2.8 19.4 

Larch 2.8 19.4 

Lodgepole pine 3.1 12.8 

Redwood 2.8 19.4 

Other hardwoods 9.2 26.7 

Unclassified & other 10.7 3.3 

Pinyon-Juniper 9.8 3.9 

Chaparral 9.8 3.9 

Nonstocked 2.6 15.2 

Pacific Southwest 

Douglas-fir 2.3 15.5 

Ponderosa Pine 2.6 15.2 

Western White Pine 2.2 11.5 

Fir-Spruce 2.6 15.2 

Hemlock-Sitka spruce 2.6 15.2 

Larch 4.6 10.8 

Lodgepole pine 2.6 15.2 

Redwood 4.4 9.7 

Other hardwoods 2.8 11.5 

Unclassified & other 9.9 3.1 

Pinyon-Juniper 15.3 3.5 

Chaparral 15.3 3.5 

Nonstocked 2.5 10.8 

* Based on data from 1997. Regions are defined in Figure 6-2 of the Land-Use Change and Forestry Chapter. 

Step 1d: Estimate Carbon in Forest Floor 

Forest floor carbon is the pool of organic carbon (litter, duff, humus, and small woody debris) above the 

mineral soil and includes woody fragments with diameters of up to 7.5 cm. To estimate carbon in the forest floor, 

equations developed by Smith and Heath (in press) were applied to the dataset described in Step 1a. Table O-1 

shows the average forest floor carbon densities by forest type, as calculated based on the 1997 data. 

Step 1e: Estimate Carbon in Down Dead Wood 

Down dead wood is defined as pieces of dead wood greater than 7.5 cm diameter that are not attached to 

trees. Down dead wood includes stumps and roots of harvested trees. To estimate carbon in down dead wood, a 

procedure similar to estimating carbon in understory vegetation was used. Down dead wood was estimated in the 

projections by using decay rates applied to logging residue, along with equations that estimate the amount of down 

dead wood from causes other than harvesting. The percentage of down dead wood carbon relative to that in live 

trees was calculated. As for the understory carbon, average values of down dead wood were derived for each forest 

type in each region. This percentage was multiplied by the live tree carbon data from the dataset described in step 

1a to calculate the total amount of carbon in down dead wood. These percentages are given in Table O-2. This 

procedure was used because detailed databases are not available for older data. By using this procedure, carbon 

estimates from historical data are consistent with carbon estimates from current Forest Inventory and Analysis data. 

O-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001 
 Step 1f: Estimate Forest Soil Carbon 

To estimate the amount of carbon in forest soils, data for soils from the surface down to a depth of 1 meter 

were obtained from the STATSGO spatial database (USDA 1991). Then a spatial data set delineating forest types 

throughout the conterminous United States (Powell et al. 1993) was overlaid onto the soil carbon data from 

STATSGO. An estimate of the average amount of carbon in the soil was then calculated for each forest type. 

Estimates included both mineral soils and organic soils.

1 Coarse roots were included with tree carbon estimates 

rather than with soils. The soil carbon estimates are given in Table O-1. These estimates were multiplied by the 

area of forest land in each forest type for all years. Thus, any change in soil carbon is a reflection of changes in the 

area of forest land or changes in forest type. 

Step 1g: Calculate Net Carbon Stock Changes 

After calculation of all forest carbon stocks, the final step was to calculate the average annual net carbon 

stock change for each forest carbon pool for the years from 1990 through 2001. The net annual stock change for 

each pool for 1987 through 1997 was derived by subtracting the 1987 stock from the 1997 stock, and dividing by the 

number of years between estimates (10 years). The stocks, by definition, correspond to the stock as of January 1 of 

the given year. The net annual stock changes for 1997 through 2001 were derived in the same way using the 1997 

and 2002 stocks. The procedure for estimating carbon stocks in 2002 is described below under the heading “Forest 

Sector Modeling Projection System.” 

Step 2: Estimate Harvested Wood Carbon Fluxes 

The first step in estimating stocks and fluxes of harvested wood (i.e., wood products and landfilled wood) 

was to compile historical data on: the production of lumber, plywood and veneer, pulp and other products; product 

and log imports and exports; and fuelwood (in terms of million cubic feet of roundwood equivalent beginning in the 

year 1900, as described in Skog and Nicholson 1998). Data were obtained from USDA (1964), Ulrich (1989), and 

Howard (2001). Projected values for wood products and roundwood use were obtained from the models used for the 

USDA Forest Service 2000 Resource Planning Act Assessment (Haynes 2002, Ince 1994, see “Forest Sector 

Modeling Projection System” below). Roundwood products include logs, bolts, and other round timber generated 

from harvesting trees for industrial or consumer use. The harvested wood-to-carbon conversion factors presented by 

Skog and Nicholson (1998) were applied to annual estimates and projections to estimate the amount of carbon in 

roundwood and wood products. The amount of roundwood consumed was categorized according to product such as 

lumber, railroad ties, and paper, because the length of time that carbon remains in these products differs 

substantially. The dynamics of carbon loss through decay or through disposal of the product is summarized as the 

half-life of each product (Skog and Nicholson 1998). The resulting estimates were applied to products to derive the 

net carbon change in wood products and landfills. Note that, unlike forest carbon stock estimates, carbon in 

harvested wood products estimates are derived as a carbon stock change. In other words, the annual roundwood 

production is a change variable already before it is converted to carbon. 

Step 3: Sum the Results from Step 1 and Step 2 for the Total Net Flux from U.S. Forests 

In the final step, net changes in forest carbon stocks are added to net changes in harvested wood carbon 

stocks, to obtain estimates of total net forest flux (see Table O-3). 

Table O-3: Net CO2 Flux from U.S. Forest Carbon Stocks (Tg CO2 Eq.) 

Description 

Forests (773.7) (773.7) (773.7) (773.7) (773.7) (773.7) (773.7) (546.3) (546.3) (546.3) (546.3) (546.3) 

Trees (469.3) (469.3) (469.3) (469.3) (469.3) (469.3) (469.3) (447.3) (447.3) (447.3) (447.3) (447.3) 

Understory (11.0) (11.0) (11.0) (11.0) (11.0) (11.0) (11.0) (14.7) (14.7) (14.7) (14.7) (14.7) 

Forest Floor (25.7) (25.7) (25.7) (25.7) (25.7) (25.7) (25.7) 29.3 29.3 29.3 29.3 29.3 

Down Dead Wood (55.0) (55.0) (55.0) (55.0) (55.0) (55.0) (55.0) (58.7) (58.7) (58.7) (58.7) (58.7) 

Forest Soils (212.7) (212.7) (212.7) (212.7) (212.7) (212.7) (212.7) (55.0) (55.0) (55.0) (55.0) (55.0) 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 

1 Organic soils, otherwise known as histosols, are soils that develop in wetland areas and have greater than 20 to 30 

percent organic matter by weight depending on clay content. 

O-5
Harvested Wood (209.0) (198.0) (202.8) (203.9) (210.5) (205.3) (205.3) (212.7) (205.3) (216.3) (209.0) (212.7) 

Wood Products (47.7) (40.7) (46.6) (54.6) (60.9) (55.0) (55.0) (58.7) (51.3) (62.3) (58.7) (58.7) 

Landfilled Wood (161.3) (157.3) (156.2) (149.2) (149.6) (150.3) (150.3) (154.0) (154.0) (154.0) (150.3) (154) 

Total (982.7) (971.7) (976.4) (977.5) (984.1) (979.0) (979.0) (759.0) (751.7) (762.7) (755.3) (759.0) 

Note: Parentheses indicate net carbon “sequestration” (i.e., accumulation into the carbon pool minus emissions or stock removal from the carbon pool). The sum 

of the net stock changes in this table (i.e., total net flux) is an estimate of the actual net flux between the total forest carbon pool and the atmosphere. Lightly 

shaded areas indicate values based on a combination of historical data and projections. Forest values are based on periodic measurements; harvested wood 

estimates are based on annual surveys. Totals may not sum due to independent rounding. 

Forest Sector Modeling Projection System 

The modeling projection system is a set of models that has been used for the USDA Forest Service, 

Resource Planning Act Assessment since the late 1980s (Figure O-1). The models include an area change model 

(Alig 1985), a timber market model (TAMM; Adams and Haynes 1980), a pulp and paper model (NAPAP; Ince 

1994) and an inventory model (ATLAS; Mills and Kincaid 1992). Many of these models are econometric models, 

designed to project the demand and supply and prices in the forest sector. Results of the projection include timber 

volume, forest areas, harvests, and primary product production. For a description of the assumptions and results of 

the modeling system, see Haynes (2002). 

The FORCARB model (Plantinga and Birdsey 1993, Heath and Birdsey 1993, and Heath et al. 1996) uses 

data on timber volume, forest areas, and harvests from the modeling system to estimate carbon in trees using 

biometrical relationships between carbon and live tree volume. FORCARB estimates carbon in all other forest 

ecosystem components, producing carbon density estimates similar to those in Table O-1 and Table O-2. The model 

WOODCARB (Skog and Nicholson 1998) uses harvested roundwood product statistics, along with end-use, decay 

rate, and duration information to estimate carbon in harvested wood. 

Figure O-1 illustrates the connections between the various models, data inputs, and data outputs that 

comprise the forest sector modeling projection system. Names of model authors are in parentheses in each model 

box to facilitate identification of model citations. Data that are external to the models are marked with double lines. 

Figure O-1: Forest Sector Modeling Projection System 

O-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001 
Figure O-1: Forest Sector Modeling Projection System

Macro Economic Forest Inventory 

Data Data 

(consumption & (1) private Timberlands 

production of wood products) (2) estimated forest growth 

parameters 

AREA 

MODELS 

(Alig) 

ATLAS 

(Mills) 

TAMM 

(Haynes/ 

Adams) 

FORCARB 

(Heath) 

NAPAP 

(Ince) 

timberland 

area 

estimates of 

fuelwood 

harvests 

annual forest growth 

timber removals 

Data for private 

Timberlands and 

some Woodlands: 

(1) area (acres) 

(2) harvested area 

and volume 

(3) growing stock 

inventory 

and increment 

Carbon stocks 

in private Timberlands 

and some Woodlands 

Carbon stocks in: 

(1) public Timberlands 

(2) orest Land 

(3) orest Land 

WOODCARB 

(Skog) 

Carbon stocks in 

harvested wood 

Historical 

harvested 

roundwood 

statistics 

Harvest, 

production, 

trade, 

end use, 

recycling 

Reserved F

Other F

Harvest data estimated Soils data from 

exogenously to (1) STATSGO database 

TAMM/ATLAS (harvests (2) FIA ecosystem data 

on public Timberlands (3) Other ecosystem data 

Reserved Forest Land, from the literature 

and Other Forest Land)